Chess is funny

Taking a break from housekeeping today, I reached the following position as Black, with the move. My question is, how would you evaluate the position?

The position arose from a French Exchange variation. I felt pretty good about my prospects here, as I was threatening to win the pawn on d4 in a few moves, and White's play felt too slow on the kingside.

After the game, I backed it up and looked at it to see where my opponent went off the rails. It turns out, it was the move 19 g4. He is now down about the equivalent of a rook! Here's the thing, and the reason I'm posting this: Black doesn't have a big knockout blow here. It's just that his play is very, very easy, and even with the sub-optimal 19...Nf4, I won when my opponent resigned on move 31 with a mate in one on the board. Black looks better, but who would think it was a crushing advantage?